Optical higher harmonic generator with temperature effecting phase matching

ABSTRACT

An optical higher harmonic generator includes a laser material, an excitation source for producing the population inversion in the laser material, and two reflectors constituting an optical resonator sandwiching the laser material therebetween. A crystal piece having the electro-optical effect and nonlinear optical effect is disposed within the resonator and a signal source applies to the crystal piece a signal having the modulation frequency and voltage necessary for locking the oscillation modes of the laser.

ilnited States Patent Uchida et a1. Feb. 22, 1972 [54] OPTICAL HIGHERHARMONIC 3,387,204 6/1968 Ashkin et a1 ..331/94.5

GENERATOR WITH TEMPERATURE 2 ammow.

EFFECTING PHASE MATCHING 3,407,309 10/1968 Miller [72] Inventors: TeijiUchida; Shogo Yoshikawa; Atsuiumi 3,444,479 5/1969 Harris et a]...33l/94.5

Ueki, all of Tokyo, Japan [73] Assignee: Nippon Electric Company,Limited, Primary Examinerkonald wibelt Tokyo, Japan AssistantExaminer-V. P. McGraw Attorney-Sandoe, Hopgood and Calimafde [22] Filed:July 22, 1969 V [211 App]. No.: 843,564 [57] ABSTRACT [30] ForeignApplication Priority Data An optical higher harmonic generator includesa laser material, an excitation source for producing the populationinversion July 23, 1968 Japan ..43/52375 in the laser material, and tworeflectors constituting an optical resonator sandwiching the lasermaterial therebetween. A

US. Cl crystal piece having the electroopfical efiect and nonlinear [58]Fieid of Search 331/94 5 optical effect is disposed within the resonatorand a signal Source pp to the Crystal piece a Signal having the modula[56] References Cited tion frequency and voltage necessary for lockingthe oscillation modes of the laser. UNYTED STATES PATENTS 3,412,25111/1968 Hargrove ..33l/94.5 2 Claims, 1 Drawing Figure 0.0. SupplyAmplifier Generator XTAL Power Supply 12 PATENTEDFEBZZ I972 3, 644. 842

0.0. Supply Amplifier Generator XTAL Power Supply 1 INVENTOI'PS TEIJI'UCHIDA SHOGO YOSHHKAWA ATSUFUMI UEKI OPTICAL HIGHER HARMONIC GENERATORWITH TEMPERATURE EFFECTING PHASE MATCHING This invention relates to ahigher harmonic generator using laser light rays and, more particularly,to such a higher harmonic generator of an ultra-high-speed light pulsetrain as includes a harmonic generating element disposed within thelaser resonator.

BACKGROUND OF THE INVENTION It has been the common practice in thegeneration of higher harmonics using laser light for the laser outputbeam to be collimated into an optically nonlinear crystal piece disposedoutside of the laser resonator. However, this type of structure is notsufficiently effective, because its harmonic conversion efficiency islow. Toimprove the efficiency, several proposals have been made. One ofthem is based on the fact that the output of the mode-locked laser ispulsive and that the conversion efficiency at the pulse peak is high.Another proposal employs an optically nonlinear crystal piece disposedwithin the laser resonator so that the high-intensity fundamental laseroscillation within the optical resonator may be directly concentratedonto the crystal piece. It is true that these measures have contributedto improve the harmonic conversion efficiency; however, it is stilldifficult to put those systems into practical use because of the highexcitation power needed for the laser material.

OBJECTS OF THE INVENTION The object of the present invention is,therefore, to provide an optical higher harmonic generator having highefficiency even for low excitation power.

Another object of the present invention is to provide an optical higherharmonic generator for generating a higher harmonic light pulse train ofultra-high-speed and narrow width.

BRIEF SUMMARY OF THE INVENTION According to the present invention, thereis provided an optical higher harmonic generator comprising a lasermaterial, an excitation source for producing the population inversion inthe laser material. two reflecting mirrors constituting an opticalresonator with the laser material disposed therebetween, a nonlinear,electro-optic crystal piece inserted between one of the mirrors and thelaser material, and a mode-locking signal source for applying to thecrystal piece a modulation signal for locking the oscillation modes ofthe laser.

The excitation energy source supplies energy to excite the lasermaterial and produce the population inversion between the energy levelsassociated with laser transition. As is well known, owing to thispopulation inversion, the laser material brings about gain for the lightbeam at the laser transition frequency. Thus, laser oscillation isproduced with the help of two mirrors disposed on both sides of thelaser material to form an optical resonator. Since the insertion of anoptically nonlinear crystal piece admits the utilization of a strongoscillating electric field in the laser resonator, as is described in anarticle by R. G. Smith et al. in Applied Physics Letters, Vol. 7, No.10, Nov. 15, 1965) pp. 256-8, the mere disposition of the crystal pieceinside of the resonator gives aconversion efficiency considerably higherthan that of the conventional harmonic generation of the externalirradiation type.

Moreover, since a mode-locking modulating signal, in the presentinvention, of a frequency approximately equal to the mode frequencyinterval determined by the length of the optical resonator is appliedfrom the signal source to a crystal having the nonlinear optical effectand the electro-optical effect, the oscillating light beam within thelaser is modulated by this applied voltage. Depending upon the relationbetween the crystal axis and the axis of the optical resonator, themodulation may be either frequency modulation or amplitude modulation.The former is described in an article by E. O. Ammann et al., IEEEJournal of Quantum Electronics," Vol. QE-l, No. 1 1 (November, 1965),pp. 263-272, and the latter is described in an article by the presentinventors in the lEEE Journal of Quantum Electronics, Vol. QE-3, No. 1(January, 1967), pp. 17-30. If this applied voltage is greater than athreshold value, the mode-locking phenomenon occurs, under whichcondition the laser output becomes a train of regularly spacednarrow-width pulses of a repetition frequency equal to consumption.Since the harmonic conversion efficiency of the optically nonlinearcrystal is in proportion to the second power of the intensity of thelaser beam, this increase in the pulse peak intensity brings about ahigh conversion output. In their paper published in Applied PhysicsLetters," Apr. 1, 1966 issue, pp. -183, M. DiDomenico, Jr. et al. reportthat the increase in the higher harmonic output is realizable by makingthe fundamental laser light pulsive resorting to the mode-lockingphenomenon. It is to be noted, however, in their experiment that theharmonic generation is performed outside of the laser resonator usingthe mode-locked laser output pulse. Also, since an additional internalmodulator for the mode-locking is not needed within the opticalresonator, the increase of the internal loss of the resonator due to theinsertion of the internal modulator and the consequent decrease in theharmonic conversion efficiency, are obviated in the present invention,making it possible to attain one of the objects of the presentinvention, the realization of the optical harmonic generator of highconversion efficiency. Also, since the light pulse is reciprocatedbetween the paired reflecting mirrors of the optical resonator due tothe mode-locking phenomenon, the higher harmonic light wave has theregularly spaced pulses of its repetition frequency equal to thefundamental pulse train. Also, because of the second power-dependentcharacteristics of the harmonic converter, the harmonic pulse widthtends to be narrower than that of the fundamental frequency pulse. Thus,another object of the present invention, i.e., the generation of theultra-high-speed narrow optical harmonic pulse is achieved.

The above mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will best be understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawing, the description of which follows, wherein theFIGURE schematically shows an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing, thepreferred embodiment of the invention comprises a YAG (yttrium aluminumgarnet) rod 1 including trivalent neodymium ion (Nd disposed on one ofthe two axes of foci of the elliptic-cylinder-shaped reflector 4. On theother focal axis, a tungsten lamp 2 is disposed which is energized bythe direct-current power supply 3. The light rays emanating from thelamp 2 are concentrated onto the YAG laser rod 1, partly after beingreflected by the reflector 4. By this irradiation, the populationinversion is produced between the laser levels F and F of neodymiumions. Since both end surfaces of the laser rod 1 are polished to formthe so-called Brewster's angle with respect to the axis of rod 1, thelaser oscillation at the wavelength of 1.06 micron is linearlypolarized. Needless to say, the axis of the resonator formed by twoconcave reflecting mirrors 5 and 6 disposed at both ends of the rod 1 isin alignment with the axis of the rod 1. In the drawing, the polarizedlaser beam has the electric fleld in the plane of the drawing. (1n thefollowing, this polarization is referred to :as polarization parallelwith the plane of the sheet of the drawing.) The reflectivity of thereflecting mirrors 5 and 6 is highest at the wavelength of thefundamental wave (1.06 micron) so as to strengthen the intensity of theoscillation. In addition, at least one of the mirrors should have hightransmissivity for higher harmonics (0.53 micron) to derive thegenerated higher harmonies. The iris 7 is inserted to suppress all themodes of laser oscillation other than the fundamental transverse mode. Acrystal piece 10 of lithium niobate (L,N,,O is inserted in the resonatoras is shown in the drawing, with its a axis oriented in parallel withthe optical axis of the resonator and with c axis normal to thepolarization direction of the laser output. With this orientation, thecrystal piece 10 has the ordinary refractive index of approximately2.236 for the laser light rays (1.06 micron) while, as to the higherharmonics (0.53 micron), the extraordinary refractive index isapproximately 2,232 at room temperature. At a higher temperature around40 C., the above-mentioned refractive indices are made equal. With thisso-called phase matching condition fulfilled, the efficient generationof the higher harmonics is realized. The temperature oven 11 iscontrolled by the power supply 12 and maintains the temperature of thecrystal piece 10 to fulfill the condition of the phase matching.

A mode-locking signal from the signal source 13 is amplified by theamplifier l4 and then applied to the electrodes attached to the crystalpiece 10 at the planes (two surfaces normal to the c axis). Thefrequency of the signal is selected approximately equal to c/2L, where Lis the optical length of the resonator and c is light velocity. With theelectric field applied in the c-axis direction, the refractive index n,of the crystal piece for the light polarized in the direction normal tothe c axis is changed by an amount /z)r -,n,, E (r equals one of theelectro-optical constants). As a result, the laser light is subjected tofrequency modulation by the modelocking signal voltage. If the appliedmode-locking signal voltage is of the order above 10 volts RMS, themode-locking phenomenon is observed, with the consequence that the laserproduces a train of sharp pulses. Owing to this pulsive oscillation, theefficiency of the higher harmonic generation is raised to make thehigher harmonic output also pulsive. In order to reduce the loss, anantireflection coating may be applied on the a planes (input and outputsurfaces) of the crystal piece 10.

While the principles of the invention have been described in connectionwith specific apparatus, it is to be clearly understood that thisdescription is made only by way of example and not as a limitation tothe scope of the invention. For example, in this embodiment, one endsurface of the YAG laser rod may be polished perpendicularly to theoptical axis, with the antireflection coating thereon. Moreover, bothend surfaces may be perpendicular to the optical axis, if theoscillation in linear polarization is assured by, for example, anoptical flat plate inserted within the resonator. Since the tungstenlamp 2 is only for exciting the laser rod 1, any other kind of lamp,such as an argon-arc lamp, or a luminous diode, may be used. Also, thereflecting cylindrical reflector 4 may be replaced by spherical orspheroidal reflectors adapted to concentrate the excitation light raysonto the laser rod 1. One of the reflecting mirrors 5 and 6 may be flator convex instead of concave. Alternatively, the reflecting mirror 5 maybe formed by evaporation on the end surface of the laser rod after beingpolished perpendicularly to its optical axis. The iris 7 may bedispensed with when the thickness of the rod 1 and the configuration ofthe resonator are suitable. The crystal piece 10 may be replaced withany other similar crystals such as barium-sodium niobate (Ba NaNb owhich have the optical nonlinearity and the electro-optical effect. Theorientation of the a and c axes is not limited to the above-mentionedexamples. Further, the modulation for mode-locking is not limited tofrequency modulation. Amplitude modulation may be adopted by rotatingthe crystal piece 10 about its a axis by 45 so as to convert a portionof the electric field of the laser beam normal to the c axis into higherharmonics. In this modification, however, the temperature of the crystalshould be finely controlled in order to prevent the loss possiblyproduced even under the no modulation condition. Instead of theantireflection coating, prisms may be attached to the crystal piece 10at both of its ends to form a Brewsters angle, which is made of materialhaving the refractive index approximately equal to that of the crystal10.

The modulation frequency for mode-locking is not restricted to c/2L. Itmay be an integral multiple of c/2L. Furthermore, the laser rod 1 may bemade of any other kind of crystal or glass capable of forming a laserrod, and the neodymium ions forming the laser-active material may be anyother activable substances such as rare earth elements. If desired, agas or liquid laser can be substituted for the solidstate material. Thelaser-active material may be excited not by the irradiation light sourceoptically, but by a DC power source electrically. Depending on thewavelength of the laser oscillation, the crystal piece 10 should besuitably selected. Phase matching may be realized not only bytemperature control but also by applying a bias voltage. The conditionof the phase matching must be taken into consideration exactly as to theinstantaneous refractive index observed at the instant when the lightpulse within the mode-locked laser passes through the crystal piece 10(the refractive index continuously varies in response to the modulatingvoltage). Therefore, although the condition of the phase matching isslightly dif' ferent from that corresponding to zero modulating voltage,the adverse effect is practically almost negligible.

What is claimed is:

1. An optical higher harmonic generator comprising: a laser material; anexcitation source for producing a population inversion in said lasermaterial; two reflectors disposed on either side of said laser materialfor constituting an optical resonator having an optical axis; a crystalpiece having the electro-optical effect and nonlinear optical effectdisposed within said resonator, said crystal piece being selected fromthe group consisting of lithium niobate and barium-sodium niobate, saidcrystal piece having an a axis parallel with said resonator optical axisand a c axis normal to the polarization direction of the laser output; asignal source for applying to said crystal piece a signal having themodulation frequency and voltage necessary for locking the oscillationmodes of the laser, and means for controlling and maintaining thetemperature of said crystal piece for effecting phase matching such thatoptical higher harmonics of the laser output are generated.

2. The higher harmonic generator claimed in claim 1, wherein one of saidreflectors is made from material having a high transmissivity at thedesired higher harmonic.

1. An optical higher harmonic generator comprising: a laser material; anexcitation source for producing a population inversion in said lasermaterial; two reflectors Disposed on either side of said laser materialfor constituting an optical resonator having an optical axis; a crystalpiece having the electro-optical effect and nonlinear optical effectdisposed within said resonator, said crystal piece being selected fromthe group consisting of lithium niobate and barium-sodium niobate, saidcrystal piece having an a axis parallel with said resonator optical axisand a c axis normal to the polarization direction of the laser output; asignal source for applying to said crystal piece a signal having themodulation frequency and voltage necessary for locking the oscillationmodes of the laser, and means for controlling and maintaining thetemperature of said crystal piece for effecting phase matching such thatoptical higher harmonics of the laser output are generated.
 2. Thehigher harmonic generator claimed in claim 1, wherein one of saidreflectors is made from material having a high transmissivity at thedesired higher harmonic.